Solitary herb growing in dense, hemispherical tussocks or dense mats, with thin pale rhizomes branching at or below ground level, and ascending to erect vegetative shoots almost without internodes between the pairs of leaves. Flowering stems, when present, ascending or erect from tussocks, up to 5–10 cm. Entire plant glabrous except for flowering stems where stems, leaves, pedicels, and bases of calyces are pubescent with 0.3–1 mm long, articulate, translucent hairs.

Leaves opposite, up to 6 × 5 mm, densely crowded on vegetative shoots, orbicular or broadly oblong with rounded apex, somewhat succulent, glabrous, bright green or reddish. Vegetative shoot apices transform into bulbils formed from two or more densely crowded pairs of leaves, leaving distinctive pairs of withered (pale) leaves in the shoot apices when detached.

Flowering stems mostly with two or more flowers in a dichasial cyme. Bracts strongly reduced, green or red, sometimes with a narrow, white hyaline margin at apex.

Flowers radially symmetric with 5 free sepals and petals. Calyx hemispherical in outline and base of calyx rounded. Sepals (3)4–6 × (1.5)2–3 mm, ovate, obtuse to subacute, with distinct, often broad hyaline margins. Petals 7–10 × 4–7 mm, ca. 2 times as long as sepals, broadly spathulate, distinctly cleft, white. Stamens 10 (number sometimes reduced). Gynoecia of 5 carpels with 5 stigmas.

Fruit a capsule but very rarely present in Svalbard and never observed with mature seeds.

Vegetative reproduction by bulbils from the shoot apices (see above), and this seems to be the only and very efficient way of reproduction in Svalbard (and perhaps in this subspecies, see Comments). The plant has been said to flower very rarely. This is not quite right; flowering is rather common but is usually triggered by shorter day length, i.e., usually commences at the end of the growing season when the midday sun is at a lower angle and the light quality changes into more near red (see Heide et al. 1990). The lack of observed flowering in C. regelii in Svalbard is probably because it flowers later than botanists are doing fieldwork. The flowers are, in principle, adapted to insect pollination. Pollen production is good but fruit ripening must be very rare in Svalbard.

Bulbils mature and are detached onwards from mid July, being spread by water, wind or animals.

The species of Cerastium can be mistaken for those of the related genus Stellaria. Cerastium has petals cleft at most to 25 %, whereas Stellaria has petals split nearly to the base (or, in species not occurring in Svalbard, without petals). When in fruit, Cerastium has a fusiform to nearly cylindrical capsule opening by 10 or rarely 6 teeth (Cerastium cerastoides), whereas Stellaria has a subglobular to short cylindrical capsule opening by 6 teeth.

Cerastium arcticum, C. alpinum, and C. regelii differ from C. cerastoides in several characters: by having five styles, a capsule with more or less skewed top opening by ten teeth, by the shoots not long and procumbent, with hairs on all sides and the leaves not upturned, and petals non-translucent white (“whole milk”). Cerastium cerastoides has three styles, a more or less straight capsule opening by 6 teeth, shoots are procumbent with hairs only on one side and leaves upturned, and the petals are translucent white (“skimmed milk”). The proposal to recognize C. cerastoides in its own genus, Dichodon, has some merit.

Cerastium arcticum, C. alpinum, and C. regelii are closely related in the so-called C. alpinum group (Hultén 1956; Böcher 1977; Brysting & Hagen 1999; Brysting & Borgen 2000; Brysting & Elven 2000; Brysting et al. 2007a, 2007b). Cerastium regelii differs in its leaves being glabrous or nearly glabrous, almost succulent, and nearly orbicular. This plant usually grows in more or less compact tussocks or mats of shortened vegetative shoots, and flowering is normally restricted to only some plants in a population and mainly at the end of the season. The two other species have much more elongate leaves, are always densely hairy, and they flower profusely in most plants of the populations throughout the season.

Moist to wet snowbeds, shallow mires, plains with abundant melt water from the permafrost or seepage from melting snow patches, river and lake margins. The species prefers fine-grained substrates (clay, loam, sand) and tolerates standing water and even inundation for protracted periods. Probably indifferent as to soil reaction (pH).

Cryophilous (preferring cold conditions). Common in all zones and sections but less frequent in the middle arctic tundra zone than in the colder zones. Common on all larger islands, including Bjørnøya, and also present on a majority of the visited smaller islands. Cerastium regelii is among the most hardy of all Svalbard vascular plants.

Cerastium regelii is circumpolar in the arctic zones, but with a racial differentiation (see Comments). Our race, ssp. caespitosum, is broadly amphi-Atlantic from N Canada and N Greenland across the North Atlantic to Svalbard (incl. Bjørnøya), Franz Joseph Land, NE European Russia and NW Siberia.

Cerastium regelii belongs to the C. alpinum group (see C. arcticum). It is much less polymorphic than the two other Svalbard species of this group (C. alpinum and C. arcticum). In Svalbard, the variation in growth form (hemispherical tussocks or more diffuse flat mats in the moss layer) seems mainly to depend on site conditions, i.e., it is modificative. The flowering seems to depend on environmental conditions in combination with light quality. In primary habitats, flowering commences very late in the season, if at all, whereas flowering may start at almost any time in the season in disturbed sites where the species is secondary (e.g., on road verges and gravel bars in rivers). Heide et al. (1990) showed a dependence on shorter day length for flowering of C. regelii.

However, at a global scale, there is an interesting variation independent of the day length factor. Nordic authors have uniformly assumed C. regelii s. str. to be the cushion plant of Svalbard. The type behind the species name is from King William Land in the Canadian Arctic Archipelago, and this type specimen (in the Oslo herbarium) shows a straggling plant with extended, procumbent shoots with distantly spaced pairs of leaves, and each shoot ending in a single flower or an open, few-flowered cyme. This plant has no parallel in Svalbard and would not easily have been identified as C. regelii by people familiar with the Svalbard plant. The original description of C. alpinum var. caespitosum was made by Malmgren in 1862, based on Svalbard plants, nearly 50 years before the first description of the species C. regelii (in 1909). Whereas Nordic and American botanists have confused the Svalbard cushion plant with C. regelii s. str., Russian botanists (e.g., Tolmachev 1971) have been aware of the difference and have accepted the Svalbard plant as C. regelii ssp. caespitosum. The two subspecies have rather different ranges: ssp. caespitosum is broadly amphi-Atlantic in the High Arctic from Banks Island in NW Canada eastwards across the Atlantic and to Taimyr; ssp. regelii is broadly amphi-Beringian in the Low Arctic from Yakutia eastwards across the Bering Straits to NE Canada. The two are found together in several places, e.g., in mixed stands on Banks Island in Canada (R. Elven observ. 1999), but without observed intermediates. However, no differences between them have been found in molecular studies (Brysting et al. 2007b).

The lack of seed-set in C. regelii ssp. caespitosum, in Svalbard and elsewhere, is probably not due to any genetic disturbance. It is rather an effect of the very late flowering (and of the bulbil reproduction being so efficient). This is supported by the high frequency of plants morphologically intermediate between the hexaploid C. arcticum and the tetraploid C. regelii. These plants also show other signs of being hybrids, not least aborting fruits and partly pollen, as are to be expected due to the difference in ploidy levels. For such hybrids to appear in such amounts as they do, C. regelii must produce quite good pollen as this species must be the male parent. Cerastium arcticum flowers throughout the season, until the snow or frost comes, and can receive and utilize pollen from C. regelii at any time. Hybrids are quite common in Svalbard, especially in disturbed sites where the flowering of C. regelii starts earlier. The type specimen of C. arcticum var. sordidum of Hultén (1956) is such a hybrid.

Böcher, T.W. 1977. Cerastium alpinum and C. arcticum, a mature polyploid complex. – Botaniska Notiser 130: 303–309.

Brysting, A.K., Aiken, S.G. & Scott, P.J. 2007(a). Caryophyllaceae. – In: Aiken, S.G. (ed.) et al., Flora of the Canadian Arctic Archipelago: Descriptions, illustrations, identification, and information retrieval. – [CD-ROM version] National Research Council of Canada, Ottawa.

Brysting, A.K. & Borgen, L. 2000. Isozyme analysis of the Cerastium alpinum – C. arcticum complex (Caryophyllaceae) supports a splitting of C. arcticum Lange. – Plant Systematics & Evolution 220: 199–221.

Brysting, A.K. & Elven, R. 2000. The Cerastium alpinum – C. arcticum complex (Caryophyllaceae): numerical analysis of morphological variation and a taxonomic revision of C. arcticum Lange s. lat. – Taxon 49: 189–216.

Brysting, A.K. & Hagen, A. 1999. Species in polyploid complexes? The Cerastium alpinum – C. arcticum complex. – Skrifter Norske Videnskaps-Akademi. Oslo, I. Matematisk–Naturvidenskapelig Klasse, n. s. 38: 183–190.

Brysting, A.K., Oxelman, B., Huber, K.T., Moulton, V. & Brochmann, C. 2007(b). Untangling complex histories of genome mergings in high polyploids. – Systematic Biology 56: 467–476.

Heide, O.M., Pedersen, K. & Dahl, E. 1990. Environmental control of flowering and morphology in the high-arctic Cerastium regelii, and the taxonomic status of C. jenisejense. – Nordic Journal of Botany 10: 141–147.

Hultén, E. 1956. The Cerastium alpinum complex. A case of worldwide introgressive hybridization. – Svensk Botanisk Tidskrift 50: 411–495.

Tolmachev, A.I. 1971. Cerastium L. – In: Tolmachev, A.I. (ed.), Flora Arctica URSS. VI. Caryophyllaceae–Ranunculaceae: 30–52.